What Does this Program Do?

Frequently, one must use or modify sections of another programmer’s code. Since the original author is often unavailable to explain his/her code, and documentation is unfortunately not always available or sufficient, it is essential to be able to read and understand an arbitrary program.

This category presents a program and asks the student to determine what the program does. The programs are written using a pseudocode that should be readily understandable by all programmers familiar with a high-level programming language, such as Python, Java, or C.

Description of the ACSL Pseudo-code

Construct	Description
Operators	! (not), ^ or ↑ (exponent), *, / (real division), % (modulus), +, -, >, =, <, >=, <=, !=
Assignment	variable = expression
Input/Output	input variable, output expression
If statement	if condition then ... end if
If/Else	if condition then ... else ... end if
For loop	for var = start to end step n ... next var
While loop	while condition ... end while
Arrays	A(i) — 0-indexed
Strings	a[i] — 0-indexed, len(a) for length
Functions	def name(params) ... end def

Sample Problems

Problem 1

What does the following program output when h = 50 and r = 10?

input h, r
b = 0
if h > 48 then
    b = b + (h - 48) * 2 * r
    h = 48
end if
if h > 40 then
    b = b + (h - 40) * (3/2) * r
    h = 40
end if
b = b + h * r
output b

Solution: This program computes an employee’s weekly salary given the hourly rate (r) and hours worked (h). The employee is paid regular rate up to 40 hours, time and a half up to 48 hours, and double for all hours over 48.

b	h
0	50
40	48
160	40
560	40

Therefore, the final value of b is 2*2*10 + 8*(3/2)*10 + 40*10 = 40 + 120 + 400 = 560.

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Problem 2

After the following program is executed, what is the final value of num?

a = "BANANAS"
num = 0; t = ""
for j = len(a)-1 to 0 step -1
    t = t + a[j]
next j
for j = 0 to len(a) - 1
    if a[j] == t[j] then
        num = num + 1
    end if
next j

Solution: The program first stores the reverse of string a into string t, then counts the number of letters that are in the same position in both strings. There are 5 such positions: 1, 2, 3, 4, and 5. So num = 5.

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Problem 3

After the following program is executed, what is the final value of C(4)?

A(0) = 12; A(1) = 41; A(2) = 52
A(3) = 57; A(4) = 77; A(5) = -100
B(0) = 17; B(1) = 34; B(2) = 81
j = 0; k = 0; n = 0
while A(j) > 0
    while B(k) < A(j)
        C(n) = B(k)
        n = n + 1; k = k + 1
    end while
    C(n) = A(j)
    n = n + 1; j = j + 1
end while

Solution: The following table traces the variables through execution:

j	k	n	A(j)	B(k)	C(n)
0	0	0	12	17	12
1	0	1	41	17	17
1	1	2	41	34	34
1	2	3	41	81	41
2	2	4	52	81	52
3	2	5	57	81	57
4	2	6	77	81	77
5	2	7	-100	81	81

Thus, C(4) = 52. This program merges two arrays in increasing order into one array until a negative number is encountered.

Video Resources